rn,flags: add ability to override resolution using a flag

Also, use the configured resolution to set it as the max received frame size.

react/features/base/flags/constants.js

@@ -81,6 +81,13 @@ export const RAISE_HAND_ENABLED = 'raise-hand.enabled';
  */
 export const RECORDING_ENABLED = 'recording.enabled';
 
+/**
+ * Flag indicating the local and (maximum) remote video resolution. Overrides
+ * the server configuration.
+ * Default: (unset).
+ */
+export const RESOLUTION = 'resolution';
+
 /**
  * Flag indicating if server URL change is enabled.
  * Default: enabled (true)